In the process of electrophotography an image is recorded in the form of an electrostatic latent image on a photosensitive member. The latent image is subsequently rendered optically visible by application of electroscopic marking particles commonly referred to as toner. The toner-based image is transferred to another substrate such as a sheet of paper and affixed thereto. The toner is commonly fixed or fused to the substrate by a combination of heat and pressure. That is, the temperature of the toner is elevated to a point at which elements of the toner coalesce and become tacky such that these elements flow into fiber or pores or otherwise along the substrate surface. Thereafter, as the toner material cools, the toner material solidifies and bonds firmly to the support member.
Conventionally, a common approach to heat and pressure fusing of electrostatic images on a support substrate such as paper involves passing the substrate with the toner images formed thereon between a pair of roller members at least one of which is heated. The heated member is commonly referred to as the fuser roller.
In the past, toner particles have been offset, i.e., transferred to the fuser roller for a variety of reasons, including insufficient heating, surface imperfections on the fuser roller or insufficient electrostatic forces to hold the toner particles against the substrate. Several solutions have been provided to address this problem. Typically, the surface of the fuser roller is coated with a low-surface energy release agent, such as silicone oil. Such release agents are transferred to the fuser roller from an oil sump via a roller assembly wherein one or more roller surfaces are wet with the agent and, through rolling action, the agent is transferred to the fuser roller. See, for example, U.S. Pat. Nos. 6,075,966 and 6,112,045 each now incorporated herein by reference. It is desirable that such assemblies, referred to as oilers, pass controlled and consistent amounts of oil, i.e., release agent, to the fuser roller.
A prior art oiler configuration is shown in FIG. 1 wherein an oil sump 2 includes a metering roller 4 positioned against a wick 6 to take up silicone-based oil (release agent fluid) along a surface 8 of the metering roller 4 from the sump 2. A metering blade 10 supported by a holder 12 is positioned against the metering roller 4 to limit the amount of oil carried along the surface 8 to the surface 14 of a donor roller 16. Transfer of oil through the wick is believed to limit streaking. The donor roller 16 is in frictional contact with the fuser roller 18 as well as the metering roller 4 such that movement of the fuser roller 18 drives rotation of the metering roller 4 to transfer the release agent from the sump to the surface of the fuser roller.
Despite numerous modifications and improvements made to such oiler systems, undesirable characteristics persist. For paper substrates it is important to transfer a uniform and consistent amount of release agent to the fuser roller surface. However, in multisheet printing operations it is common for the release agent transfer rate to begin at three to four times the desired rate and to substantially decline after the first ten to twenty sheets are processed. This surge of release agent may be attributed to several factors.
Residual fluid is often left on the fuser roller surface from prior duplication runs. The amount of such fluid depends in part on the split ratio between rollers 4, 16 and 18. With a simple 50 percent split in fluid volume between rollers, the residual release agent fluid on the fuser roller can rise to four times the desired steady state rate.
In addition, if the oiler remains idle for a significant time interval, e.g., five to ten minutes, some fluid will migrate to the metering blade 10 by capillary forces. With this accumulation in place, when the oiler is next engaged, a surge of fluid, e.g., tens of mgs, will be transferred to the fuser roller and ultimately to the substrate.
Another factor affecting the volume of release agent transferred is the oil viscosity which, varies substantially with temperature fluctuations. Thus, in systems which require thermal fusing of the toner, temperature variations are to be expected and such variations will have an influence on viscosity. Predictably, the temperature of the metering roller is relatively low at the beginning of a run and increases as each sheet is processed during the run. While it is somewhat difficult to quantify the viscosity variation, limited tests indicate that normal heating can alter the viscosity to the point where, if other variables remain constant, the fluid transfer rate may at least double.
One other variable affecting the oil transfer rate is the uncontrollable variation in roller speeds, particularly the speed of the metering roller which is driven by the donor roller. When there is too much oil on the adjoining surfaces, substantial slippage occurs which, in turn, results in slower movement of the metering roller. As the metering roller speed decreases, the amount of oil transferred to the donor roller also decreases. It should also be noted that, when there is a speed differential between the rollers, a drag force may persist which force can accelerate wear of the fuser roller.
The aforementioned variables are believed to result in non-uniform and unpredictable oil transfer rates. Further, although the oil transfer rates may be established through design, such rates are fixed, i.e., not adjustable, for individual designs.
Another difficulty with existing systems relates to required blade tolerances. That is, if the blade is not made with sufficient precision, defects along the blade edge result in non-uniform oil transfers across the rollers. Such transfers are known to create image streaks.
It is desirable to provide improved methods and improved systems which control the amounts and uniformity of a release agent. Such improvements would result in more satisfactory image reproduction and lower maintenance of associated equipment. It is also desirable to adjustably control the rate of fluid transfer from the sump to the fuser roller. In conventional designs, one or more parameters may be adjusted to control the transfer rate, but because these are fixed for each design the transfer rate is not user adjustable.